Modernize GetLogicalDrives docs

[MSDN-WhiteKnight]

• Add information about Unix behaviour
• Replace mention of floppy drives that are no longer common
• Make descriptions of similar methods in Directory and Environment class more consistent

[dotnet-policy-service[bot]]

Tagging subscribers to this area: @dotnet/area-system-io

[learn-build-service-prod[bot]]

Learn Build status updates of commit e05fb50:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System.IO/Directory.xml	:white_check_mark:Succeeded	View
xml/System/Environment.xml	:white_check_mark:Succeeded	View

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